Part 3: Internetworking

Internet architecture, addressing, encapsulation, reliable transport and

the TCP/IP protocol suite

Internetworking: concepts, architecture and protocols

Motivation, architecture, routers, TCP/IP protocols, internet reference

model

Motivation• A large organisation will use several

networking technologies• Inter-organisational communication is

significant• Universal service - any two computers

should be able to communicate• However, different network technologies

cannot just be wired together

Internetworking

• Interconnect heterogeneous networks and provide universal service– Hardware: routers connect different networks– Internet protocols: provide universal service by

creating a single virtual network

Internet architecture

• Although a single router can connect many networks, most organisations use multiple routers

Virtual network• The illusion

that there is a single universal network

Internetworking protocols

• The TCP/IP Internet Protocols– begun in the 1970s– The Internet has emerged into the public

domain in the 1990s– Controlled by the Internet Engineering Task

Force (IETF)

Internet Reference Model

Host computers

• TCP/IP used the term host computer to refer to any system that connects to an Internet and that runs applications

• Both hosts and routers use TCP/IP protocol software

IP: Internet protocol addresses

Uniform addressing, the IP address hierarchy, address classes, dotted

decimal notation, special addresses, routers and addresses, address

resolution

Uniform addressing

• Internet protocols deal in packets and provide uniform addressing

• The addressing scheme is defined in software and is used transparently by applications

• Internet addressing is specified in the IP protocol

• Each host is assigned a unique 32 bit address

The IP address hierarchy

• Each 32 bit address is divided into two parts– prefix: physical network to which the host is

attached - the network number– suffix: a host attached to a given physical network

• Prefixes are coordinated globally and suffixes locally

Classes of IP address

• Size of prefix and suffix determines maximum number of networks and maximum number of hosts per network

• IP defines different classes of address with different sized prefixes and suffixes

• The first four bits of the address specify its class

The five classes of IP address

Dotted decimal notation

• Makes it easier to for humans to use addresses (names are also possible)

Classes and dotted decimal

Division of the address space

• Public Internet network numbers are assigned by Internet Service providers (ISPs) and these are coordinated by the Internet Assigned Number Authority

An addressing example

Classless Addressing

• The Internet is running out of addresses• Allow division between prefix and suffix to appear at

an arbitrary boundary• Consider network with only 9 hosts

– Only need four bits for host suffix– Class C (smallest) address uses 8 bits for host suffix– Can subdivide a class C address into 16 addresses with a

28 bit prefix and 4 bit suffix• Extend dotted decimal notation

– 193.68.138.0/28, 193.68.138.16/28, …,193.68.138.240/28

Special IP addresses

Routers and IP addressing

• Routers are assigned two or more IP addresses

• So are multi-homed computers

Binding protocol addresses• An Internet packet passes through a series of

routers – each hop takes it over a particular network,

either to a specific computer on that network or to the next router

– in either case, the sending router has to map between the protocol (IP) address and a hardware address

– this is called address resolution

Address resolution techniques

• Table lookup• Closed-form computation• Message exchange

– send message to specific server computers– broadcast message, only the required computer

responds

Pros and cons of techniques

Address resolution protocol

• TCP/IP defines the Address Resolution Protocol (ARP) which defines the format of resolution requests and responses

• This technique is usually combined with local caching of hardware addresses

Dynamic Host Configuration Protocol (DHCP)

• Special DHCP server that assigns IP addresses to hosts

• Newly booted machine broadcasts a DHCP discover packet

• DHCP server sends back an IP address– Permanent IP addresses

• Manually assigned by administrator– Automatic IP address from a pool of addresses to be

allocated on demand• Leased for a finite period of time

DHCP Operation

• DHCP server does not need to be on the same network as the host

Summary

• Uniform addressing• Address classes• Dotted decimal notation• Classless addressing• Special IP addresses• Address resolution (ARP) and ssignment (DHCP)